1. Field of the Invention
The present invention relates to a chemically-amplified negative resist composition used for processing a semiconductor and a photo mask blank, and a resist patterning process using the same.
2. Description of the Related Art
In recent years, as an integrated circuit progresses toward a high integration, a further finer patterning is required. When a resist pattern of 0.2 μm or less is processed, as a usual manner, a chemically amplified resist composition in which an acid acts as a catalyst has been used. As the exposure light source, a high energy beam such as an ultraviolet beam, a far-ultraviolet beam and an electron beam has been used, and especially, electron beam lithography, which is used as ultrafine processing technique, has become indispensable as a method of processing a photo mask blank in producing a photomask used for semiconductor manufacturing.
Polymers having a large amount of aromatic skeleton with acidic side chain, such as polyhydroxystyrene, have been favorably used as a resist composition for KrF excimer laser, but not for ArF excimer laser because they exhibit large absorption to light whose wavelength is approximately 200 nm. Due to high etching resistance, however, this type of polymer essentially serves as a useful resist composition for electron beam and as a resist composition for extreme ultraviolet (EUV) to form a finer pattern than a processing limit using an ArF excimer laser.
There are two types of resist compositions: a positive resist composition whose exposed area is dissolved and a negative resist composition whose exposed area is processed into a pattern, and they are selected according to a resist pattern to be required. A chemically-amplified negative resist composition normally contains a polymer that dissolves in an aqueous alkaline developer, an acid generator that generates an acid by decomposition of a resist film by exposure and a crosslinking agent that insolubilizes a polymer in a developer by forming a crosslink between polymers using an acid as a catalyst (a polymer and a crosslinking agent can be integrated). In addition, a basic compound for controlling diffusion of an acid generated by exposure is usually added thereto.
Many negative resist compositions using a phenol unit as an alkali-soluble unit composing the above polymer that dissolves in an aqueous alkaline developer have been investigated for exposure particularly with KrF excimer laser beam. In fact, these negative resist compositions have not been used for ArF excimer laser beam because a phenol unit has no transmittance to an exposure light whose wavelength is 150 to 220 nm. Recently, however, they have received a growing technical attention as a negative resist composition for EB and EUV exposure, which are used in exposure method for forming a finer pattern, and examples thereof are disclosed in Patent Documents 1, 2 and 3.
Meanwhile, to control resist sensitivity and pattern profile, various improvements have been achieved according to the way materials used in a resist composition are selected, combined and processed. One of its effective improvements is to control diffusion of an acid that has a significant impact on resolution of a chemically-amplified resist.
Acid diffusion control agent is substantially essential component to control acid diffusion and improve resist performance. Development of an acid diffusion control agent has conventionally been discussed in various manners, and amine and acidulous onium salt are commonly used. As for an example of the acidulous onium salt, Patent Document 4 describes that the addition of triphenylsulfonium acetate enables a favorable resist pattern without T-top shape, line width difference between a dense pattern and an isolated pattern, and standing wave to be formed. Patent Document 5 describes improvement in sensitivity, resolution and exposure margin by addition of a sulfonic acid organic salt or a carboxylic acid organic salt. In these technologies, a strong acid generated from other photo acid generators by exposure (sulfonic acid) is exchanged with an acidulous onium salt to produce weak acid and strong acid onium salt. Accordingly, by substituting a strong acid of high acidity (sulfonic acid) with a weak acid (carboxylic acid), acid decomposition reaction of an acid-labile group is controlled to achieve a smaller (controlled) acid diffusion distance. Apparently, it functions as an acid diffusion control agent.
However, use of the-described resist composition containing a carboxylic acid onium salt or a fluorocarboxylic acid onium salt for patterning can cause a higher LER. Thus, development of an acid diffusion control agent capable of reducing LER has been desired.